What is the smallest difference in lumens that the average human eye can see?

Answer 1

Actually, the ability of the human eye to differentiate levels of luminous flux is not best measured in an absolute value of lumens. Rather, the human eye can distinguish light sources with a ratio of luminous flux of about 2:1. So if one source of light has between half and twice as much luminous flux as a second source, they will actually appear to be just about as bright as each other. This is why LEDs are sorted into bins with a maximum brightness ratio of about 1.5:1. They can be used together and they will have the same apparent brightness.